1. Field of the Invention
The present invention relates to a unique construction of a rasp of the type used for smoothing a cut edge of material, such as a cut edge of gypsum board. The invention also relates to a method of smoothing a cut edge of material, such as gypsum board.
2. Description of the Prior Art
In the building trade, gypsum board, sometimes referred to as wallboard or drywall, is utilized extensively as the primary surfacing material forming the interior walls and ceilings of buildings. Although sold in other sizes, the wallboard is most often sold commercially in four foot by eight foot rectangular sections. Very frequently these sections must be cut to size to fit into corners and to accommodate window and door openings. Wallboard is typically cut to size using a conventional box cutter or utility knife.
In cutting wallboard it is very important not to cut the sheet of material too small, as this would leave gaps that require an extensive amount of time and labor to fill. As a consequence, drywall installers often tend to cut the wallboard pieces slightly oversize and then reduce the size slightly along the cut edge.
One implement that is widely utilized commercially for removing material from a cut edge of wallboard is an article known as a wallboard rasp. A conventional wallboard rasp is formed of two major components, namely a handle and an abrading element. The handle is very often fabricated from wood and is formed as a generally rectangular block, typically about six or seven inches in length, about two inches in width and one inch in thickness. A shallow trough is formed lengthwise down the center of the underside of the block while a pair of longitudinally extending narrow slots or crevices are formed along the sides of the block. The crevices are designed to receive the longitudinal edges of the abrading element.
The abrasive portion of the abrading element may be formed of an expanded metal sheet. Such an expanded metal abrasive sheet is formed from a thin sheet of metal in which a multiplicity of short slits are cut in parallel rows. The ends of the slits are offset from each other in adjacent rows. The sheet metal is then pulled laterally from opposite sides in a direction perpendicular to the alignment of the slits. This causes the slits to open and for the metal between the slots to twist so that the narrow strips of metal between the slits are twisted through large acute angles relative to their original alignment prior to expansion of the metal. The final orientation of the metal strips approaches an orientation perpendicular to their original coplanar alignment. The resulting structure bears somewhat of a resemblance to a screen, but has sharp metal edges facing outwardly at large acute angles relative to the plane of expansion of the metal sheet.
To create the abrasive element of a conventional wallboard rasp a section of the expanded metal sheet is cut to a size slightly shorter than the length of the wooden block handle and wide enough to cover the underside of the wooden block handle and extend a short distance up its sides. Sheet metal angles are then spot welded to the longitudinally extending side margins of the sheet of expanded metal. The spot welds connect the side margins of the sheet of expanded metal to one leg of each of the metal angles. The other leg of each metal angle is directed inwardly into one of the two narrow slots on the opposite sides of the wooden block handle. The abrasive element is thereby secured to the underside of the wooden block handle, but extends over the longitudinal edges of the underside at the transition between the underside and the side walls of the block.
A conventional wallboard rasp is constructed and used in a manner quite similar to a conventional sanding block. That is, the user grips the smooth upper portion of the wallboard rasp and runs the exposed edges of the expanded sheet metal section across a cut edge of wallboard. The sharp, exposed edges of the expanded sheet metal section are sharp enough to abrade away both the paper on the outer surface of the wallboard, and also the gypsum interior. The cut section of wallboard can then be abraded to the desired size and the cut edge of the wallboard will be smoothed in the process.
Conventional wallboard rasps perform their intended function quite adequately. However, they are also the source of many painful injuries. Since the expanded metal abrasive element is wrapped over the outer edges of a supporting block, there is a considerable exposure of sharp surface area that creates painful injuries with only light contact to the skin of a user. As a consequence, users must proceed slowly and carefully when utilizing this implement. Even so, many injuries result.
Furthermore, the transversely cut ends of the expanded metal present an additional source of injury. While the sheet of expanded metal forming the abrasive element is normally cut to a length slightly shorter than the supporting wooden block that serves as a handle so that the ends of the wooden block extend longitudinally beyond the ends of the expanded metal abrading element, the ends of the abrading element are still quite dangerous. This results from cutting the sheet of expanded metal to size. When the strips of metal that form the lattice to create a multiplicity of grid openings in the expanded metal are cut, short barbs are left that project outwardly at the cut ends of the expanded metal sheet. These barbs can sometimes become bent forwardly and project beyond the end of the underside of the wooden block handle. This exposure creates a considerable safety hazard. The ends can be capped with a folded over marginal strip of metal which is spot welded to encase therewithin the cut barbs at the ends of the sheet of expanded metal. However, this adds to the cost of fabrication of the product.
A further safety hazard is presented by the exposed longitudinal edges at the bottom of a conventional wallboard rasp. In such a conventional structure the sheet of expanded metal is wrapped at right-angles about the supporting wooden block at the longitudinal edges of the underside of the wooden support block. Due to the reflex angles through which the expanded metal sheet is bent, the longitudinally extending edges of the wallboard rasp at the transition between the underside and side walls of the tool are particularly dangerous due to their significant exposure. Even though users normally handle the tool with gloves, any abrasive contact between the exposed portions of the expanded metal rasp element and the fabric of the glove can cause the expanded metal to easily slice through the glove and create painful injuries. The incidences of injuries utilizing conventional wallboard rasps has been frequent enough so that many wallboard installers will not use them, even though they perform their intended function quite admirably.